add interlinks plugin for markdown

[neo2k]

Manage frequently used links in config file.

Config example:

interlinks.example=http://www.example.de

Usage in markdown:

[Normal boring link](http://www.example.com). 
But this is a [cool link](example>) that links to this site.

generates:

<a href="http://www.example.com">Normal boring link</a>. 
But this is a <a href="http://www.example.de">cool link</a> that links to this site.

Inspired bei interlinks plugin from pelican website generator.

[jonbullock]

Thanks for the contribution, I'll leave this open until plugin support has a bit more structure to it as I'd like to include this functionality and potentially not just for Markdown.